Loss of Par3 promotes prostatic tumorigenesis by enhancing cell growth and changing cell division modes.

Although cell polarity plays an important role in epithelial tumorigenesis, the consequence of polarity protein loss in prostatic tumorigenesis and the underlying mechanisms remain unclear. Using conditional knockout mouse models, we found in the current study that loss of polarity protein Par3 increases prostatic epithelial cell growth, elevates symmetrical cell divisions in basal cells, and randomizes spindle orientation in luminal cells, causing the development of high-grade prostatic intraepithelial neoplasia (PIN). Mechanistically, loss of Par3 dissociates the Par3/merlin/Lats1 complex, consequently inhibiting phosphorylation of Lats1 to attenuate the Hippo pathway. Furthermore, attenuated Hippo pathway enhances nuclear translocation of Yes-associated protein (YAP), which promotes cell proliferation and symmetrical cell divisions through transcriptional activation of Ki-67 and Sox2. In addition, Lats1 dephosphorylation impairs its interaction with G protein signaling modulator 2 (GPSM2, which is also known as LGN) that causes randomization of spindle orientation in luminal cells. Interestingly, co-deletion of Par3 and Lats1 for complete blockade of the Hippo pathway in mice results in prostate tumor initiation, whereas co-deletion of Par3 and YAP for disrupting YAP nuclear translocation reverses the phenotypes to a relatively normal state. Therefore, our findings highlight combination of Par3 loss and blockade of the Hippo pathway as a novel mechanism for prostatic tumorigenesis.